1. Field of the Invention
The present invention relates to activated carbon that is excellent in terms of not only adsorptivity but also desorptivity of organic gas. In addition, the present invention relates to a canister using, as an adsorbent, such activated carbon that is excellent in terms of adsorptivity and desorptivity of organic gas.
2. Background Art
Because of high volatility, gasoline used as fuel for automobiles is vaporized in a fuel tank when a vehicle is running or is left parked in strong sunlight, and the produced gasoline vapor is released into the atmosphere. Gasoline vapor is also generated during refueling.
To prevent gasoline vapor from being released to the outside of a vehicle, a canister in which activated carbon as an adsorbent adsorbs gasoline vapor is provided for the vehicle. For instance, when an internal combustion engine for automobiles contains a canister capable of adsorbing and desorbing fuel vapor, emission of fuel vapor that has evaporated from the fuel tank of a vehicle to the outside is prevented. In such a canister, fuel vapor generated after a vehicle stops running is temporarily adsorbed. In addition, the adsorbed fuel component is desorbed together with newly adsorbed vapor so as to be subjected to combustion treatment in the internal combustion engine during the subsequent running.
This adsorbent in a canister also serves to adsorb gasoline vapor generated during refueling. The adsorbed gasoline vapor is desorbed (purged) from activated carbon in accordance with engine rotation, directed into the engine via a suction pipe with air taken in from the outside, and burned therein.
Meanwhile, adsorption performance of activated carbon as an adsorbent for fuel gradually deteriorates due to repetition of adsorption and desorption of organic gas (vapor) such as gasoline. When the amount of vapor generated in a fuel tank exceeds the adsorption performance of activated carbon in a canister upon vapor generation, vapor is released to the outside of the vehicle, resulting in generation of an abnormal odor, etc.
Hitherto, activated carbon in a canister has been used without being subjected to any treatment, or polarity has been imparted to such activated carbon for the purpose of improving adsorption performance, for example.
In general, activated carbon is produced in a manner such that a starting material thereof is subjected to carbonization followed by activation. Activated carbon has pores for adsorption of gasoline vapor. Activation is a step of developing such pores and controlling pore diameters. Activated carbon used for a canister has been required to have pores with large opening diameters (20-50 Å) for adsorption and desorption of gasoline vapor. Such pores with large diameters have been created by carrying out advanced activation or chemical activation under more stringent conditions than those applied to usual forms of activation treatment.
As described above, a desired adsorbent used in a canister is an adsorbent that adsorbs an organic gas such as gasoline and desorbs the gas after a certain period of time for adsorption so that large amounts of adsorption and desorption can be achieved. This is because such adsorbent can maintain its performance without deterioration even after being repeatedly used.
Even if gasoline vapor adsorbed on activated carbon in the canister is subjected to desorption (purging) with air (suction air), a part of the adsorbed gasoline vapor remains on activated carbon, and with a rise of temperature when a vehicle is left parked, a residual gasoline vapor is desorbed from activated carbon and released outside the vehicle.
Especially in the case of conventional activated carbon, which has a wide pore diameter distribution, pores with small opening diameters have higher adsorptivity than pores with large opening diameters, making it difficult for the adsorbate to get desorbed (purged). Therefore, the adsorbate (residue) that remains adsorbed in pores with small opening diameters exists in a large quantity, resulting in a leakage of gasoline vapor upon increase of ambient temperature. Thus, the amount of hydrocarbon released (exhausted) into the atmosphere (bleed emissions) increases when a vehicle is left parked, which has been problematic.
In JP Patent Publication (Kokai) No. 2004-256335 A, it was found that the residue remains adsorbed in the pores with small opening diameters of activated carbon used as an adsorbent. The document discloses that a method for producing activated carbon wherein pores with small opening diameters are occluded is effective in reducing residual amounts of gasoline vapor, such that leakage of such gasoline vapor is minimized, and is also effective in inhibiting bleed emissions of gasoline vapor into the atmosphere. Specifically, an organic compound is adsorbed on activated carbon having pores with a wide range of diameters so as to selectively occlude pores with small diameters.
Meanwhile, when vapor enters in pores of activated carbon in a canister, vapor forms a primary adsorption layer on the surfaces inside the pores via Van der Waals force due to molecular interaction. Further, liquid layers formed with vapor as a result of the Kelvin effect due to capillary condensation are adsorbed on the primary adsorption layer in a sequential manner. Upon engine purging, an adsorbate in the upper layer starts to become desorbed.
However, upon engine purging, an adsorbate in the primary adsorption layer is unlikely to be desorbed due to a relatively strong bond between the adsorbate and activated carbon. In particular, a component of the adsorbate having a high boiling point is more unlikely to be desorbed than a component having a low boiling point. Thus, the component having a high boiling point tends to remain in the pores. That is, when a component of an adsorbate is desorbed from the primary adsorption layer, such component having a low boiling point is desorbed from the pores, and such component having a high boiling point is adsorbed in the pores after desorption of the component having a low boiling point, such that the volume of the component having a high boiling point gradually increases. As a result, deterioration of activated carbon progresses. In addition, the component having a high boiling point has a large molecular weight so that it occupies a large part of the volume inside each pore. Thus, the volume inside each pore is reduced and adsorption performance further deteriorates.
When using a conventional porous adsorbent made of activated carbon or the like, a substrate or a surface functional group of such adsorbent influences whether hydrophobic or hydrophilic adsorption takes places. In addition, a desired adsorbent may be an adsorbent that achieves large amounts of adsorption and desorption after adsorption for a certain period of time followed by desorption. In such case, needless to say, the surface area, pore distribution, and the like of the adsorbent largely influence amounts of adsorption and desorption. Hydrophobicity and hydrophilicity of an adsorbent can be advantageous or problematic factors. Hydrophobicity and hydrophilicity of an adsorbent influence the condition of adsorption and desorption where the amount of adsorption is small with respect to an adsorbate, or where the amount of adsorption is large with respect to an adsorbate and the amount of desorption is small.
When considering the hydrophobicity and hydrophilicity of the aforementioned adsorbent, the adsorbent is selected depending on the hydrophobicity or hydrophilicity of an adsorbate of interest. When the adsorbent is repeatedly used for adsorption and desorption, an adsorbent that achieves a large amount of desorption after adsorption (a small residual amount of adsorbate left after desorption) is required. When an adsorbate and the surface of an adsorbent share the same properties, the amount of adsorption increases while the amount of desorption decreases.
For instance, as in the case of activated carbon used for a canister, when hydrocarbon components are adsorbed on or desorbed from activated carbon, a pore distribution that results in an increased amount of adsorption causes adsorption retentivity to increase, leading to the decreased amount of desorption. Thus, the amount of adsorption significantly decreases in the subsequent adsorption. In addition, a pore distribution that results in an increased amount of desorption causes adsorption retentivity to decrease. Thus, the amount of adsorption significantly decreases or the hardness of activated carbon is reduced upon advanced activation whereby pore sizes become enlarged. The above phenomena have been problematic.